a) Field of the Invention
This invention relates to a method for manufacturing a silencer (or muffler) for use in an internal combustion engine of an automobile, as an example. More particularly, it is concerned with a method for producing such silencer constructed with a multi-perforated inner tube and an outer tube, between which a sound-absorbing material such as glass wool, etc. is filled. The invention is also concerned with an apparatus for manufacturing such silencer.
b) Background of the Invention
There have so far been proposed various types of such silencer (or muffler) constructed with the multi-perforated inner tube, the outer tube, and the sound-absorbing material such as glass wool, etc. is filled in the space between these inner and kouter tubes. One example of such silencer is shown in FIGS. 6 and 7 of the accompanying drawing.
FIG. 6 illustrates a sound absorbing type silencer to be used as a pre-chamber or a sub-muffler to be disposed on the way of the exhaust system the internal combustion engine for automobiles, the main body M of which is constructed in an inner/outer double structure composed of a multi-perforated inner tube T1 made of a punched metal, etc. with multiple holes being formed therein and an outer tube T2, a sound-absorbing material S such as glass wool, etc. being filled in the space between both inner and outer tubes T1, T2. In the drawing, B1 and B2 designate end plates for closing the end parts of the inner and outer tubes T1, T2, while P1 and P2 refer to exhaust pipes connected to both end parts of the silencer main body M1.
FIG. 7 also illustrates a baffle type silencer (or muffler) to be used as the main muffler, etc. in the exhaust system such as described above, in which diffuser pipes D1, D2 for communicatively connecting a plurality of expansion chambers C1 to C3 defined in the silencer main body M are constructed with the multi-perforated inner tube T1 and the outer tube T2, and in which sound-absorbing material S such as glass wool, etc. is filled between the inner tube T1 and the outer tube T2.
In manufacturing the silencer constructed with the inner tube T1, the outer tube T2, and the sound absorbing material S interposed between both inner and outer tubes T1, T2 such as described above, it is a general practice to wind the sound -absorbing material S, in advance, on and around the outer periphery of the multi-perforated inner tube T1, as shown in FIG. 8 for example, after which this inner tube T1 is inserted into the outer tube T2.
Also, in order to exhibit favorable sound-muffling performance, the abovementioned sound-absorbing material S such as glass wool, etc. is required to be uniformly filled in between the inner and outer Lubes T1, T2 at a predetermined density. However, since the sound-absorbing material S has elasticity, hence it is bulged outward more largely than the inner diameter of the outer tube T2, as shown in FIG. 8(a), it is difficult to insert the inner tube T1 covered with the sound-absorbing material S into the outer tube T2. If and when such inner tube is forced to push into the outer tube, there is such apprehension that fluctuation occurs in the filling density of the sound-absorbing material S, or the sound-absorbing material deforms prominently in the direction opposite to the direction of insertion of the inner tube T1 due to friction between these two sound-absorbing material and the outer tube, as shown in FIG. 8(b), with the consequence that it juts out from the end part of the outer tube T2, or the sound-absorbing material S melts out when the end paltes B1, B2 are welded to the end parts of the outer tube T2, to bring about deterioration in the quality of the sound absorbing material, or defects in such welding of the end plates, or else. Further, when the winding quantity of the sound-absorbing material S on and around the inner tube T1 is reduced for its easy insertion into the outer tube T2, the packing density of the sound-absorbing material becomes short with the consequence that no satisfactory silencing performance becomes able to exhibit.
Therefore, it has conventionally been a practice to impregnate the sound-absorbing material S with a binding agent by spraying or dipping, at the time of winding the sound-absorbing material S on and around the outer periphery of the inner tube T1, thereby making the outer dimeter of the sound-absorbing material S to be substantially equal to the inner diameter of the outer tube T2, followed by insertion of the inner tube T1 into the outer tube T2. However, when such binding agent as mentioned above is used, the sound-absorbing material S becomes solidified, which compels the silencing performance to be reduced. And yet, there would accrue various inconveniences such that the manufacturing cost of the silencer becomes increased, because impregnation and drying steps for the binding agent is required, and so forth.